Compressor with capacity control

ABSTRACT

Compressor containing a pressure regulating system ( 8 ) comprising an inlet valve ( 9 ); a piston ( 23 ) in a cylinder ( 24 ) connected thereto; a bridge ( 14 ) of this inlet valve ( 9 ) with a non-return valve ( 16 ) in it, characterized in that the piston ( 32 ) is a double-acting piston; the cylinder ( 24 ) on the side of the piston ( 23 ) turned away from the inlet valve is connected to a part ( 13 ) of the rotor chamber ( 2 ) situated near the inlet valve ( 9 ) via a pipe ( 28 ); and the cylinder ( 26 ) on the other side of the piston ( 23 ) is connected to the above-mentioned part ( 13 ) of the rotor chamber ( 2 ) and the non-return valve ( 16 ) via a pipe ( 29 ).

The present invention concerns a compressor containing a compressorelement which is provided with a rotor chamber onto which are connectedan inlet pipe and an outlet pipe, a reservoir in the outlet pipe and apressure regulating system comprising an inlet valve erected in theinlet pipe, a piston which is connected to the inlet valve and which canbe moved in a cylinder, a bridge bridging said inlet valve and in which,between the inlet pipe and the rotor chamber, are successively erected agas stream limiter and a non-return valve which only admits gas into therotor chamber, and a gas pipe connecting the reservoir to the part ofthe bridge situated between the gas stream limiter and the non-returnvalve, and a relief valve erected in said gas pipe.

Depending on certain parameters such as operating pressure, temperature,leakages, delivery or the like, or depending on a specific compressedair network and the length of the pipes, or also, depending on the typeof application or the like, a certain type of compressor element willhave to be selected which has to meet the total consumption under theworst conditions.

In reality, however, there will be variations in certain of theabove-mentioned parameters. When the compressed air consumption is lowerthan the production, the pressure in the pipes will rise. When theoperational pressure is reached in the network of pipes, the productionof compressed air will be stopped in order to prevent unacceptable highpressures being created. After a while, the pressure in the pipes willreduce again due to leakages, consumption or the like and, depending onthe application, pressure will have to be built up again in order toprevent the operational pressure from dropping under an unacceptablelimit.

For compressors with rotors, such as screw-type compressors, thepressure-regulating system described in the first paragraph, also calleda load and relief system, is one of the most frequently used regulatingsystems to allow for a production of compressed air from 0 to 100% witha minimum of energy loss.

In the case of such compressors, the variations in the consumption ofcompressed air are adjusted by opening and closing the inlet valve andthe pressure relief in the reservoir.

As soon as the operational pressure reaches a certain level, thepressure regulating system makes sure that the inlet valve of thecompressor element is closed. The supply of inlet air is in this mannerreduced to zero percent, and the compressor element will run idle. Theair supply at the outlet pipe, in particular at the reservoir which isusually erected in it, is stopped. When the inlet valve is closed, thepressure regulating system simultaneously activates a time switch whichmakes sure that the drive of the compressor element keeps on working fora certain period.

If no specific pressure difference occurs after this period, thepressure regulating system will order the drive to be stopped. If,however, a pressure difference occurs after the aforesaid period, thecompressor element will keep on working and the pressure regulatingsystem will order the inlet valve to be opened again, so that pressurecan be built up again.

When the drive has come to a standstill and the pressure level in theoutlet pipe is too low, the pressure regulating system will order thecompressor element to be started, whereby the inlet valve is opened.

With known compressors of the above-mentioned type, the pressureregulating system contains a strong spring, built-in in the cylinder andpushing on the side of the piston which is turned towards the inletvalve, while the cylinder chamber situated on the other side of thepiston is connected to the reservoir via a control line, equipped withan electromagnetic control valve.

When the rotors are driven at the initial start-up, the control valve isnot excited, and the pressure in the reservoir is close to theatmospheric pressure. The relief valve in the gas pipe is open and,under the influence of the spring on the piston, the inlet valve isclosed. Due to the underpressure created in the rotor chamber, a smallair flow will flow from the inlet pipe through the bridge, over the gasstream limiter and the non-return valve, to the rotor chamber,sufficient to provide for an increase of pressure in the reservoir.

A continuous air flow is created between the bridge, the rotor chamber,the reservoir and over the pneumatic relief valve which has been openedby the built-up pressure, and then back to the bridge. When the drive isready to run at full load, the control valve is excited, as a result ofwhich the relief valve goes back into the closed position, and the spaceabove the piston in the cylinder is simultaneously put under pressure,and the spring force is overcome, such that the inlet valve is opened.The production of compressed air now amounts to 100%.

When there is more production of compressed air than demanded, and theset pressure in the reservoir is maximal, the excitation of theelectromagnetic control valve is stopped, as a result of which this isclosed again. The space above the piston is connected to the atmospherevia the control valve, and the relief valve is opened again. As aresult, the inlet valve is closed again under the influence of thespring, and the reservoir is vented via the relief valve, the gas pipeand the bridge.

After this venting, the pressure is stabilised at the pressure for idlerunning, which is sufficient to provide for the injection of lubricationliquid on the rotors. A small amount of air bridges the inlet valve andis sucked into the rotor chamber via the bridge and the non-returnvalve. The production of compressed air is reduced to a minimum and thecompressor turns without producing anything.

As there is a strong spring in the inlet valve, special precautions haveto be taken. The mounting and dismounting of the inlet valve is notwithout any danger because of said spring. Because of the spring, theinlet valve is also relatively expensive. In order to be able to relievethe spring pressure of the inlet valve, an expensive electromagneticcontrol valve with a large passage diameter is required.

When the relief valve and the inlet valve are controlled simultaneously,malfunctions sometimes occur.

The invention aims a compressor which does not have the above-mentioneddisadvantages and which is thus relatively inexpensive, allows for aneasy mounting and dismounting of the inlet valve and allows for areliable control of the inlet valve.

According to the invention, this aim is reached in that the piston is adouble-acting piston which divides the cylinder in two closed cylinderchambers, in that the cylinder chamber, on the side turned away from theinlet valve, is connected to a part of the rotor chamber situated nearthe inlet valve via a pipe, and in that, on the other side of thepiston, the cylinder chamber is connected to a part of the rotor chambersituated near the inlet valve and to the non-return valve via a pipe.

Thus, there is no action of a spring on the piston anymore.

The pipe connecting the cylinder chamber on the side which is turnedaway from the inlet valve to a part of the rotor chamber situated nearthe inlet valve may as such form the connection between the piston andthe inlet valve, and it may for example consist of a stem provided witha duct over its entire length.

The relief valve may then, as in the known pressure regulating systems,be a pneumatic valve which is controlled by a pipe connected directly tothe reservoir, a control line having a preferably electromagneticcontrol valve in it which is also connected to said reservoir, and aspring.

In order to better explain the characteristics of the invention, thefollowing preferred embodiment of a compressor according to theinvention is described as an example only without being limitative inany way, with reference to the accompanying drawings, in which:

FIG. 1 schematically represents a compressor according to the invention;

FIG. 2 schematically represents the pressure regulating system of thecompressor from FIG. 1 during the start-up;

FIG. 3 schematically represents the pressure regulating system of thecompressor from FIG. 1, but when running idle;

FIG. 4 represents a section of a practical embodiment of a part of thepressure regulating system from FIGS. 2 and 3.

The compressor which is schematically represented in FIG. 1 is ascrew-type compressor which mainly comprises a compressor element 1which is provided with a rotor chamber 2 onto which are connected aninlet pipe 3 on the one hand and an outlet pipe 4 on the other hand, andin which are erected two screw rotors 5 working in conjunction which aredriven by a motor 6, a reservoir 7 which is erected in the outlet pipeand a pressure regulating system 8.

As is also represented in the FIGS. 2 and 3, the pressure regulatingsystem 8 has an inlet valve 9 with a valve element 10 which works inconjunction with a valve seat 11 in the valve housing 12.

There where the inlet pipe 3 opens into the rotor chamber 2, the latterforms a protruding inlet chamber 13 in which the valve element 10 is inthe open position.

The inlet valve 9 is bridged by a bridge 14 in which the inlet valve 3and the inlet chamber 13 are successively provided, a gas stream limiter15 and a non-return valve 16 which only allows a gas stream into theinlet chamber 13.

The part of the bridge 14 situated between the gas stream limiter 15 andthe non-return valve 16 is connected to the reservoir 7 via a gas pipe17. In this gas pipe 17 is erected a pneumatic relief valve 18 having anopen position and a closed position.

The relief valve 18 is controlled by an electromagnetic control valve 19in a control line 20 which is connected to the reservoir 7 or, asrepresented in FIG. 1, between this reservoir 7 and the relief valve 18,to the gas pipe 17 on the one hand, and which is connected to the farend of the relief valve 18 on the other hand, onto which also acts aspring 21. On the other far end, which is connected to the reservoir 7or the part of the gas pipe 17 situated between the relief valve 18 andsaid reservoir 7 via a pipe 22, the pressure acts in the reservoir 7.

In one position, the control valve 19 opens the control line 20, and inanother position, it closes off said control line 20 on the side of thereservoir 7, while it connects the control line to the atmosphere on theside of the relief valve 18.

The pressure regulating system 8 further comprises a double-actingpiston 23 which can be moved in a cylinder 24 and which divides thiscylinder 24 in two closed cylinder chambers 25 and 26. The piston 23 isconnected to the valve element 10 of the inlet valve 9 by means of astem 27, such that they move together.

The cylinder chamber 25 on the side of the piston 23 which is turnedaway from the inlet valve 9 is connected to the inlet chamber 13 via apipe 28, whereas the other cylinder chamber 26 is connected to the partof the bridge 14 situated before the non-return valve 16 and the gasstream limiter 15 via a pipe 29 or, as is represented in FIG. 1, via thenon-return valve 16 to the part of the gas pipe 17 connected onto thispart of the bridge 14.

When the compressor is initially started up, the pressure in thereservoir 7 is close to the atmospheric pressure. The control valve 19is not excited and the part of the control line 20 connected to therelief valve 18 is connected to the atmosphere such that, under theinfluence of the spring 21, the relief valve is closed and closes offthe gas pipe 17.

The motor 6 must easily reach its maximum speed. A small air flow flowsout of the inlet pipe 3 via the bridge 14 into the rotor chamber 2,which is sufficient to build up a pressure in the reservoir 7.

When the pressure being built up in the reservoir 7, which acts on therelief valve 18 via the pipe 22, neutralises the operation of the spring21, the relief valve 18 will go into its open position, as representedin FIG. 2.

Thanks to the open relief valve 18, the pressure being built up in thereservoir 7 is also available in the cylinder chamber 26, as a result ofwhich the piston 23 is being held in the top position, so that the inletvalve 9 remains closed. There is an underpressure in the inlet chamber13, as a result of which the valve element 10 is drawn open, but thisforce is compensated because the same underpressure prevails in thecylinder chamber 25 via the pipe 28. The diameter of the valve element10 and the diameter of the piston 23 are selected such that the vacuumforces exerted upon it compensate each other.

There is a continuous air flow from the reservoir 7, over the openrelief valve 18 and the bridge 14 and the compressor element 1, and backto the reservoir 7.

When the motor 6 is ready for a full load, the electromagnetic controlvalve 19 is excited, as a result of which the control line 20 opens, asrepresented in FIG. 3.

The pressure of the reservoir 7 now acts, via the control line 20 on theone hand and via the pipe 22 on the other hand, on the relief valve 18,and the spring 21 will push the relief valve 18 into the closedposition, as is also represented in FIG. 3.

As a result, the reservoir 7 is no longer vented via said relief valve18 and the gas pipe 17. The cylinder chamber 26 is no longer connectedto the reservoir 7, but to the inlet chamber 13 via the bridge 14 wherethere is an underpressure which also prevails in the cylinder chamber 25via the pipe 28. Vacuum forces draw the valve element 10 into the openposition. The result of the forces on the piston 23 and on the valveelement 10 is a force which makes the inlet valve 9 open.

The compressor operates at full load, and the production of air amountsto 100%.

When the production of compressed air exceeds the demand, the pressurein the reservoir 7 will rise, and as soon as it reaches a specificvalue, the pressure regulating system will stop the excitation of thecontrol valve 19, so that this control valve 19 interrupts the controlline 20 again and brings the part thereof which is connected to therelief valve 18 in connection with the atmosphere.

As described for the start-up, the relief valve 18 will as a resultthereof go into its open position, and the inlet valve 9 will closeagain. The condition as represented in FIG. 2 is created again.

The reservoir 7 is vented via the gas pipe 17, over the open reliefvalve 18 and the bridge 14, partly over the gas stream limiter 15 in theinlet pipe 3, and partly over the non-return valve 16 in the inletchamber 13.

After this venting, the pressure will stabilise at the pressure for idlerunning, which pressure is sufficient to provide for the injection oflubrication liquid on the rotors.

The compressor again not only sucks a small amount of air through thebridge 14, which amount of air flows back to the bridge 14 via the gaspipe 17. The compressor in this manner keeps on running idle, withoutdelivering compressed air.

After a pre-programmed length of time, the pressure in the reservoir 7is measured by the pressure regulating system 8 and, when there has beenno pressure drop, also the motor 6 will be stopped.

In case of a pressure drop in the reservoir 7 as a result of adiminution of air, the motor 6 will keep on running and the pressureregulating system 8 will excite the control valve 19 again, so that thecondition as represented in FIG. 3 is created again, with an openinlet-valve 9 in the above-described manner.

By making use of the above-described pressure-regulating system 8, it ispossible to use a inexpensive electromagnetic control valve 19 with asmall passage, and the relief valve 18 will be more reliable as the airflow, through the control valve 19, only has to control said reliefvalve 18 and not the piston 23 in the cylinder 24.

Moreover, it is not necessary to use a heavy spring acting on thepiston, which is safe and non-expensive, and as a result of which thecylinder 24 can be made compact.

How the cylinder 24 and the inlet valve 9 as a whole can be made verycompact in practice is represented in FIG. 4.

The valve housing 12, the cylinder 24 and a far end 3A of the inlet pipe3 have been united into a single housing 30 which is fixed on the rotorhousing 32 by means of bolts 31.

Also the inlet chamber 13 is present in this global housing 30 and formsa whole with an opening 33 in the rotor housing 32.

The two far ends of the bridge 14 are also ducts 14A and 14C provided insaid body 30 and opening on the side of the far end 3A of the inlet pipe3 in relation to the valve element 10, in the inlet chamber 13respectively.

The gas pipe 29 is formed of a duct 29 provided in said housing 30connecting the cylinder chamber 26 with a bridge 14 between duct 14B and14C.

In this compact embodiment, the pipe 28 is formed of the above-mentionedstem 27 upon which the piston 23 and the valve element 10 are fixed, andwhich is provided with a duct 34 over its entire length which opens intothe cylinder chamber 25 on the one hand, and into the inlet chamber 13or opening 33 on the other hand.

It is clear that the gas which is compressed in the compressor must notnecessarily be air. It may also be another gas, such as a gaseouscooling medium.

The present invention is by no means limited to the above-describedembodiment given as an example and represented in the accompanyingdrawings; on the contrary, such a compressor can be made in differentshapes and dimensions while still remaining within the scope of theinvention.

1. A compressor, containing a compressor element, and comprising: arotor chamber connected to an inlet pipe and an outlet pipe; a reservoirin communication with the outlet pipe; a pressure regulating systemincluding an inlet valve associated with the inlet pipe; a pistonconnected to the inlet valve and which is movable in a cylinder to openand close the inlet valve without the use of a spring acting on thepiston; a bridge bridging said inlet valve and in which, between theinlet pipe and the rotor chamber, are successively mounted a gas streamlimiter and a non-return valve which only admits gas into the rotorchamber; a gas pipe connecting the reservoir to a part of the bridgesituated between the gas stream limiter and the non-return valve; and arelief valve associated with said gas pipe, wherein the piston is adouble-acting piston which divides the cylinder into first and secondclosed cylinder chambers; the first cylinder chamber, on a first side ofthe piston facing away from the inlet valve, is connected to a part ofthe rotor chamber located near the inlet valve upstream the rotorchamber via a first pipe, wherein the connection is always open; and ona second side of the piston, the second cylinder chamber is connected toa part of the rotor chamber situated near the inlet valve and to thenon-return valve via a second pipe.
 2. The compressor according to claim1, wherein the first pipe connecting the first cylinder chamber on thefirst side which is turned away from the inlet valve to a part of therotor chamber situated near the inlet valve forms the connection betweenthe piston and the inlet valve.
 3. The compressor according to claim 2,wherein the connection between the piston and the inlet valve comprisesa stem provided with a duct extending over an entire length of the stem.4. The compressor according to claim 1, wherein the relief valvecomprises a pneumatic valve which is equipped with a spring and which isconnected by a pipe which is directly connected to the reservoir and acontrol line which is also connected to said reservoir via a controlvalve.
 5. The compressor according to claim 4, wherein the control valveis an electromagnetic valve.
 6. The compressor according to claim 1,wherein the inlet valve includes a housing forming a common housing withthe cylinder.